The critical mass curves of an equilateral cylindrical reactor as a function of moderation were calculated by graphical methods for H: U/sup 235/ atomic ratios of 25 down io 1.4. The critical assemblies were built up of small cubes. These cubes were of two types, H-cubes and U-cubes. The H-cubes were small blocks of polyethylene (CH/sub 1.92/) approximately 1" on the edge. The U plastic cubes (U-cubes) contained a mixture of UF/sub 4/ (U/sup 235/) isotopic concentration of 9.3% and polytetrafluoroethylene (CF/sub 2.02/) pressed together to form a material having an over-all density of 4.73 g/cm/sup 3/. The infinite cylinder and slab dimensions as functions of moderation were approximated for the low values of H: U/sup 235/ atomic ratio. (C.J.G.)